The present invention relates to a method for programming MOS and CMOS ROM memories. In particular, programming is performed by making permanently non-conductive or switched off all the cells to be tied to a preset logic state.
As is known, ROM (Read Only Memory) cells are extensively used to store permanent data and are manufactured individually or integrated with microprocessors and microcontrollers in MOS or CMOS technology. Since read-only memories must be "customized" (i.e. programmed according to the required application), the need is felt to program said memories at an advanced stage of production so as to reduce the delivery times of the devices programmed with the content requested by the user. It is furthermore known that ROM memories are generally constituted by arrays of cells formed by N-channel or P-channel transistors often having a CMOS read circuitry.
Various methods are currently used to program ROM memories. In particular, one of said methods uses the active-area mask as a programming mask. However, this method allows programming at an initial stage of the production of the devices, so that other programming methods, performed at later stages, are generally preferred. Thus, according to another solution (in the case of memory cells manufactured using N-channel transistors, which are preferred in view of the higher mobility of the electrons), boron is implanted at such a dose as to raise the threshold voltage of the transistors of the array, which are selected by means of an appropriate programming mask, above the supply voltage. This solution cannot be applied, in any case, to VLSI devices with thin gate oxides (less than 350.ANG.) since boron implanting would lower the breakdown voltage to unacceptable values.
Other solutions have therefore been proposed which allow to raise the threshold of the programming cell above the supply voltage while keeping the breakdown voltage of the source and drain junctions above the supply voltage. According to a known solution, boron is implanted at a lower dose so as to not lower the breakdown voltage of the junctions below the supply voltage while raising the threshold voltage almost sufficiently. In particular, this method comprises a programming masking, etching of the oxide, grown after the source/drain implanting, covering the cells to be programmed, and subsequent boron implanting. A re-oxidation step is then performed, producing a reduced growth of oxide on the un-programmed transistors still covered with oxide, while on the programmed uncovered cells the re-oxidation reduces and narrows the polysilicon gate regions and raises the edges of the polysilicon gate regions due to the growth of oxide below said edges, thus separating the source and drain junctions from the gate and further raising the threshold of the programmed cell, raising it above the supply voltage. However this solution, too, is not free from disadvantages, since the re-oxidation step occurs at low temperature (800.degree. C.) for a relatively long time so as to avoid threshold shifts and other doping-agent diffusion effects and entails a deviation from the standard manufacturing processes currently in use.
Still another solution is to implant the boron in the source region and subsequently diffuse it below the gate. In this manner the threshold voltage can be raised above the supply voltage without lowering the breakdown voltage, since the latter is affected only by the channel concentration proximate to the drain. However this process, too, is not always satisfactory, and in particular it cannot be applied to VLSI devices with common-source cell arrays.